Switched state fiber optic gyroscope

ABSTRACT

An improved fiber optic gyroscope with a substantial improvement in dynamic range and scale factor stability over a basic fiber optic gyroscope is disclosed. In a preferred embodiment of the invention the detected output of the gyroscope is demodulated to yield first and second voltage levels respectively proportional to preselected sine and cosine components of the detected output. These first and second voltage levels are respectively switched between a feedback network which stabilizes the drive level and a rate sensing circuit which senses the rotation rate. Such switching occurs when operational states are switched in order to stabilize the drive level from the feedback network and to provide the most sensitive one of the first and second voltage levels to the rate sensing network.

BACKGROUND OF THE INVENTION

The present invention relates to fiber optic gyroscopes and particularlyto a constant accuracy, high dynamic range fiber optic gyroscope.

The Sagnac phase shift between two counterpropagating beams is the basisfor all optical gyroscopes, although it is detected in a variety ofways. In the ring laser gyroscope and various "closed loop" opticalgyroscopes, the scale factor (e.g. counts per unit rotation rate) isfixed by the area of the optical medium, whereas the phase reading fiberoptic gyroscope (FOG) has a scale factor which increases with increasinglength of optical fiber. Thus, development of low-loss fibers holds thepossibility of extremely sensitive gyroscope operation, and has made theFOG a competitive optical gyroscope. Although most of the first decadeof FOG development has concentrated on improving sensitivity, attentionis now turning to other difficulties associated with the FOG.

Principal among the difficulties associated with the FOG are the "scalefactor" and the "dynamic range" problems. The scale factor problemrefers to the fact that the electrical output of the fiber opticgyroscope is not always the same for a given input rotation. The dynamicrange problem refers to the fact that there are limitations to the upperrotation rate (and hence the ratio of upper rate to minimum detectablerate). The "scale factor" being discussed here is not the optical scalefactor relating the Sagnac phase shift φ_(s) to the rotation rate Ω, butan "electrical" scale factor which arises from converting the opticalphase shift into an electrical signal. Such scale factor and dynamicrange difficulties will be explained more fully by now referring to theconventional or "minimum configuration" fiber optic gyroscope (FOG)shown in FIG. 1 (and described by S. Ezekiel, et al. in their article,Fiber Optic Rotation Sensors, Tutorial Review, in Fiber Optic RotationSensors, published by Springer, NY (1982)).

The fiber optic gyroscope of FIG. 1 includes a first directional coupler13 having input ports 15 and 17 and output ports 19 and 21, a seconddirectional coupler 23 having input ports 25 and 27 and output ports 29and 31, and a fiber optic coil 33 having opposite ends 35 and 37respectively coupled to the output ports 29 and 31 of the coupler 23.The fiber optic coil 33 comprises N turns wound on a plan area A. In atypical application the fiber optic gyroscope of FIG. 1 is mounted on arotating platform (not shown) with the axis of symmetry of the coil 33parallel to the axis of rotation of the platform so that the rotationrate Ω of the platform can be sensed.

Light from an optical source 39, such as a laser or super-luminescentdiode, propagates through the coupler 13 to the coupler 23, which splitsthe light into two substantially equal beams I₁ and I₂. The beams I₁ andI₂ respectively enter the opposite ends 35 and 37 of the fiber opticcoil 33, with beam I₁ propagating through the coil 33 in a clockwisedirection and beam I₂ propagating through the coil 33 in acounter-clockwise direction. After these two counter-propagating lightbeams I₁ and I₂ have traversed the coil 33, they reenter the coupler 23through its output ports 31 and 29 and, upon recombining in the coupler23, interfere with each other. This light interference is related to therate of rotation Ω of the coil 33. For example, if the gyroscope isrotating at a rate Ω, the beams I₁ and I₂ undergo a non-reciprocalSagnac phase shift, φ_(s), where ##EQU1## for a source 39 of wavelengthλ, where c is the velocity of light.

The combined interfering beams combine in coupler 23 and the resultantbeam propagates into the coupler 13. A portion of that resultant beam isdirected by the coupler 13 to a photodetector 41, where the intensity

    I=I.sub.o (1+cos φ.sub.tot)                            (2)

is detected and amplified by an amplifier 43. The intensity I representsthe standard interferometric "fringe" pattern for the totalnon-reciprocal phase shift φ_(tot), where I_(o) is the peak acexcursion.

If nothing else perturbs the fiber optic coil 33, φ_(tot) =φ_(s), and inprinciple this signal could be detected to determine the rotation rate.However, a practical problem is that the sensitivity of the gyroscope,≈ΔI/Δφ_(s), goes to zero at low rotation rates. To avoid this, a timevarying phase shift or modulation is usually applied, often by winding apart of the coil 33 around a piezoelectric transducer (PZT) 45. When thePZT 45 is driven sinusoidally at a frequency ω and drive level oramplitude A by an oscillator or PZT driver 47, a non-reciprocal phaseshift is added to the Sagnac phase shift, φ_(s) :

    φ.sub.tot =φ.sub.s +ηcos ωt.             (3)

where η=2 sin ωτ/2 and τ is the transit time of the coil 33. As a resultof the non-linear phase applied and the non-linear interferometerresponse of Equation (2), the output of the amplifier 43 contains anabundance of harmonics of the PZT drive frequency ω and the idealvoltage output of the amplifier 43 is given by:

    V=I.sub.o {(1+J.sub.o (η)cosφ.sub.s)+α.sub.1 J.sub.1 (η)sinφ.sub.s cosωt+α.sub.2 J.sub.2 (η)cosφ.sub.s cos 2ωt+ . . .}               (4)

where the α_(i) 's depend on electrical bandwidth and so forth, and theJ_(i) 's are Bessel functions.

The above voltage output of the amplifier 43, as shown in equation (3),is applied to a mixer circuit 49 where it is heterodyned with a portionof the output ω from the oscillator 47 to produce a voltage level V(ω)proportional to sinφ_(s).

By choosing η≈1.84, the maximum value of J₁ (η) is obtained, and if thegyroscope output (at the output of amplifier 43) at frequency ω isdetected, the voltage level at the output of the mixer 49 is

    V(ω)=α.sub.1 I.sub.0 J.sub.1 (η)sinφ.sub.s( 5)

where V(ω) is a voltage amplitude which reflects the sign of φ_(s) andfor which the sensitivity ΔI/Δφ is greatest at small values of φ_(s).There are two types of problems associated with Equation (5).

A first problem is that the sensitivity of the gyroscope decreases asφ_(s) increases and actually goes to zero when φ_(s) reaches ±90°. Inother words, because the slope of the sine function (sin φ_(s)) goes tozero at ±90°, small changes in the measured level of V(ω) correspond tovery large changes in the imputed φ_(s), and thus in the imputedrotation rate. Since the fiber optic gyroscope of FIG. 1 is not reliablenear or past a point where the slope of sin φ_(s) goes to zero (±90°),the conventional fiber optic gyroscope is limited to a maximum Sagnacphase shift of ±90°. This restricts the maximum rotation rate to thatvalue of Ω which corresponds to φ≈90° and, therefore, limits the overall"dynamic range" of the gyroscope to one-fourth of a fringe.

A second problem is that the output level, V(ω), also depends on J₁ (η).Unintentional changes in the level of η will cause J₁ (η) to change and,hence, cause the scale factor of the gyroscope to drift. Changes in ηcan occur due to amplitude drift in the oscillator 47 that drives thePZT 45. Such changes in η can be controlled by monitoring the level ofthe output of the oscillator 47. To attain scale factor stabilitycomparable to current sensitivities (in parts per million) requires anamplitude control of 0.1%. Such an output level is not difficult tocontrol. However, the electrical drive amplitude is not the only factorwhich influences η. Changes in the mechanical properties of the PZT 45with temperature, changes in how the fiber optic coil 33 loads the PZT45, creeping in the fiber glass (not shown) on the PZT 45, relaxation ofthe adhesive bonds (not shown) between the coil 33 and PZT 45, andslowly changing static stresses on the PZT 45 are a few of the possiblesources of error that could change η, even if the amplitude of theoscillator 47 (which drives the PZT 45) were held perfectly constant.Since some of the above mentioned sources of error can not be modelled,the scale factor stability of the gyroscope of FIG. 1 will be limited tothe level of certainty to which all such error sources are identifiedand stabilized.

One attempt to rectify the above-described problems has been disclosedby K. Bohm, et al. in their article, Direct Rotation-Rate Detection Witha Fibre-Optic Gyro By Using Digital Data Processing, 19 ElectronicsLetters 997-999 (1983). In this article, the first, second and fourthharmonics are detected by means of a 10-step digitization of the outputwaveform which is used to extract the three amplitudes. The first andsecond harmonic components are used to contruct an arctangent, while thesecond and fourth harmonics are used to hold η constant. This techniqueis an improvement over the usual method because it does take cognizanceof the fact that η can change and it increases the dynamic range of thegyroscope by looking at both the sin φ_(s) and cos φ_(s) components.However, by virtue of the fact that this technique stabilizes η bylooking only at cos φ_(s) terms, this technique can not develop anaccurate error signal when cos φ_(s) =O. In addition, the 10-stepdigitization process is expected to be less accurate, as well as 10times slower, than a technique in which V(ω) or V(2ω) are firstprocessed as analog signals and then digitized.

OBJECTS OF THE INVENTION

One object of the invention is to provide an improved fiber opticgyroscope.

Another object of the invention is to provide a state-switching fiberoptic gyroscope.

Another object of the invention is to provide a fiber optic gyroscopewhich has a much larger dynamic range of operation over the usual fiberoptic gyroscopes.

Another object of the invention is to provide a fiber optic gyroscopewhich reduces the scale factor and dynamic range problems associatedwith the well-known "minimum configuration" fiber optic gyroscope.

Another object of the invention is to provide a fiber optic gyroscopewhich automatically switches the state of the gyroscope between twostates as a function of the rotation rate of the gyroscope in order tokeep the sensitivity and accuracy of the gyroscope high for rotationrate sensing while allowing the stabilization of the drive level.

Another object of the invention is to provide a state-switching fiberoptic gyroscope which always uses the more sensitive voltage for sensingthe rotation rate at the same time it uses the potentially higheramplitude voltage as a feedback signal to stabilize the drive level.

Another object of the invention is to provide a multistate fiber opticgyroscope which automatically selects any of a plurality of states as afunction of the rotation rate.

A further object of the invention is to provide a two-state fiber opticgyroscope which, during each of the two-state operations, provides apeak-to-peak detection of the detected signal for normalizing therotation rate sensing signal at the same time that the drive level issuch as to cause η in Equation (3) to be greater than π.

SUMMARY OF THE INVENTION

These and other objects are achieved in the present invention byproviding an improved fiber optic gyroscope which comprises: means fordemodulating the detected output of the gyroscope to yield V(ω) andV(2ω) voltage levels respectively proportional to the sin φ_(s) and cosφ_(s) of that detected output; and a logic system for switching aswitching network during a first state or mode to pass V(2ω) as afeedback signal to a feedback loop to stabilize the drive level of apiezoelectric transducer (PZT) (or any other transducer) at a firstpredetermined amplitude and to pass V(ω) as a rate sense signal into arotation rate detection system to sense the rotation rate, and during asecond state or mode to pass V(ω) as the feedback signal to the feedbackloop to stabilize the drive level of the PZT at a second predeterminedamplitude and to pass V(2ω) as the rate sense signal into the rotationrate detection system to sense the rotation rate. The rate sense signalbeing used at any given time is normalized by the output of apeak-to-peak detector and decoded in the logic system to provide theleast significant bits (LSB) of the rotation rate. The LSB represent anangle of Sagnac phase shift between approximately -45° and +45°. Inaddition, each time that the rate sense signal exceeds a preselectedabsolute value, the logic system switches the switching network tochange from one state to the other state, and also increments (ordecrements) in the logic system the most significant bits (MSB) of therotation rate by a number corresponding to a Sagnac phase shift of 90°.The total rotation rate at any given time corresponds to the arithmeticsum of the LSB and MSB.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects, features and advantages of the invention, aswell as the invention itself, will become better understood by referenceto the following detailed description when considered in connection withthe accompanying drawings wherein like reference numerals designateidentical or corresponding parts throughout the several views, andwherein:

FIG. 1 is a schematic block diagram of a prior art fiber opticgyroscope;

FIG. 2 shows ideal response curves for a two-state gyroscope;

FIG. 3 illustrates response curves useful in explaining the invention;

FIG. 4 is a schematic block diagram of a preferred embodiment of theinvention; and

FIG. 5 is a TABLE useful in explaining the operation of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In essence, the present invention performs two important operations. Ina first operation, the FOG of the invention looks at both sine andcosine terms in the Bessel series expansion shown in Equation (4). Thiskeeps the sensitivty and accuracy high while allowing the stabilizationof η, the drive level. In a second operation, the FOG of the inventionnormalizes the interferometer signal V, as shown in Equation (4), in away which takes account of all the perturbations acting on I_(o) bydirectly sampling their effects.

Before the schematic block diagram of the preferred embodiment of theFOG invention of FIG. 4 is discussed, it should aid in understanding theinvention to first examine the basic concepts involved in the invention.

As for the above-mentioned first operation, FIG. 2 shows sin φ_(s) andcos φ_(s) curves that ideally could be used in a two-state fiber opticgyroscope. One (or more) of the odd harmonics in Equation (4) isprocessed (to be explained) to yield an essentially DC voltage V (ω)that is to sin φ_(s), while one (or more) of the even harmonics inEquation (4) is processed (to be explained) to yield an essentially DCvoltage V(2ω) that is proportional to cos φ_(s). As will be explained inrelation to FIG. 4, one of these V(ω) and V(2ω) voltages will be sensed,as a function of the rotation rate, in order to automatically determinethe rotation rate. However, for the sake of clarity, only the sin φ_(s)and cos φ_(s) curves of FIG. 2 will be discussed at this time.

At low rotation rates of the platform, where the Sagnac phase shiftφ_(s) of the gyroscope is between approximately -45° and +45° (-π/4 to+π/4 radians), the sin φ_(s) curve is used to determine the rotationrate Ω of the platform since it has the greater slope of the two curvesand thus the greatest accuracy between those limits of φ_(s). Thegyroscope will be said to be in a STATE 1 (S1) mode of operation whenthe sin φ_(s) curve is being used to determine the rotation rate. Thegyroscope remains in STATE 1 as long as φ₅ is between -π/4 and +π/4.Between these limits the gyroscope essentially behaves exactly like aconventional FOG does in determining the rotation rate.

If the rotation rate were to increase the Sagnac phase shift 100 _(s) toan angle greater than +45° (or +π/4), the gyroscope would switch to thecos φ_(s) curve and use that cos φ_(s) curve to determine the rotationrate. The gyroscope will be said to be in a STATE 2 (S2) mode ofoperation when the cos φ_(s) curve is being used to determine therotation rate. The gyroscope remains in STATE 2 as long as φ_(s) isbetween +π/4 and +3π/4. As shown in FIG. 2, the cos φ_(s) curve has thegreatest slope, and hence the greatest accuracy, when φ_(s) is between=+π/4 and +3π/4.

If the rotation rate continues to increase, then every additionalincrease of π/2 in the Sagnac phase shift φ_(s) will cause anotherswitch in STATES. Thus, the gyroscope would switch from STATE 1 to STATE2 when φ=π/4, from STATE 2 back to STATE 1 when φ=3π/4, from STATE 1back to STATE 2 when 0=5π/4, from STATE 2 back to STATE 1 when φ=7π/4,from STATE 1 back to STATE 2 when φ=9π/4, and so forth. High rotationrates above a φ_(s) =0° would be sensed by counting (in increments of90° or π/2) the number of times that the STATES 1 and 2 are switched. Asshown in FIG. 2, the gyroscope would be in STATE 1 when φ_(s) is between-π/4 and π/4, in STATE 2 when φ_(s) is between π/4 and 3π/4, in STATE 1when φ_(s) is between 3π/4 and 5π/4, in STATE 2 when φ_(s) is between5π/4, and 7π/4, in STATE 1 when φ_(s) is between 7π/4 and 9π/4, and soon. Note that the heavy lines of the above-defined portions of the sinφ_(s) and cos φ_(s) curves of FIG. 2 correspond to the greatestsensitivity of those curves when used in a gyroscope to sense therotation rate of the platform.

Such an operation of the gyroscope of the invention would provide theaccuracy of the conventional FOG of FIG. 1 over a relatively wide rangeof φ_(s) angles since, as shown in FIG. 2, φ_(s) would effectively betranslated (during each of the STATES 1 and 2) into a range between -π/4and π/4 while the number of times that the STATE of the gyroscope isbeing switched is being counted. This procedure can continue until themagnitude of the Sagnac phase shift φ_(s) becomes comparable to thephase coherence of the light source being used. However, even for a lowcoherence super-luminescent diode source, the above-described techniqueof counting the STATE switchings could extend the dynamic range of thegyroscope of the invention by two orders of magnitude over theconventional FOG of FIG. 1.

In the above discussion it was assumed that the sin φ_(s) and cos φ_(s)curves of FIG. 2 are well behaved. However, in actual operation thereare unavoidable deviations from such ideal curves. For instance, thecoherence of the light source being used will make the sin φ_(s) and cosφ_(s) curves in FIG. 2 damp out over a characteristic phase differenceof φ=2πL_(coh) /λ, where L_(coh) is the coherence length and λ is thewave length of the light source. The damped curves will result in everincreasing errors in the computed rates for the conventional FOG ofFIG. 1. To avoid these errors, a normalization operation must beintroduced to effectively divide Equation (4) by I_(o) to compensate forany changes in the amplitude of I_(o).

This normalization operation is the second important operation performedby the present invention and will be explained more fully in relation toFIG. 4. This normalization operation departs from the usual practice ina conventional FOG, such as the one shown in FIG. 1, of driving themodulator 45 at the η corresponding to the maximum of J₁. Instead, thenormalization operation of the present invention drives the PZT 45A(FIG. 4) to create an amplitude of η greater than π(=3.14). By equation(4), this assures an excursion over more than an entire fringe and,thus, I₀ can be measured directly by finding the peak-to-peak voltage ofthe waveform. Thus, by finding the peak-to-peak voltage of the waveform,all of the factors which contribute to changes in I_(o) can be sampledand measured.

However, correcting for changes in I_(o) in the above-described mannerintroduces a possible error source in the drive level or J_(i) (η). Inthe conventional FOG of FIG. 1 this drive level error is minimized,since driving at the peak of J₁ (η) makes the FOG vulnerable to changesin η only in the second order. In contrast, overdriving the PZT 45A(FIG. 4) permits varitions in η to affect the output drive level in thefirst order. A solution to this problem lies in driving the modulatoreither at the zero of J₂ (when in STATE 1 and sampling J₁ sin φ_(s)) orat the zero of J₁ (when in STATE 2 and sampling J₂ cos φ_(s)). Thus, inSTATE 1, J₁ sin φ_(s) is used to find the rotation rate, while J₂ cosφ_(s) is used to provide an error signal to establish the level of ηwhich corresponds to the zero of J₂. Similarly, in STATE 2, J₂ cosφ_(s)is used to find the rotation rate, while J₁ sin φ_(s) is used to providean error signal to establish the level of η which corresponds to thezero of J₁. In both cases above, the output (although dependent onchanges in η in first order) is stabilized by means of a readily derivederror signal fed to an appropriate servo loop.

The above-discussed operation of the gyroscope of the invention isbasically illustrated in FIG. 3, which plots the amplitude of the outputvoltage against each of φ_(s) and η. As the Sagnac phase shift φ_(s)increases from zero (during STATE 1), the modulation is driven at thezero of J₂ (i.e. j₂ ], so that the sin φ_(s) term can be sensed and theproper angle between -π/4 and π/4 can be determined. When the Sagnacphase shift φ_(s) increases past π/4 a switch is made to STATE 2, themodulation level is changed so as to be driven at the zero of J₁ (i.e.j₁), the cos φ_(s) term is sensed and the proper phase shift determined.As the STATES 1 and 2 are switched back and forth, provision is made tocount the number of switchings and π/2 multiples of φ_(s) arearithmetically added to the proper angle along one of the lines 51 and53 to determine the rotation rate.

Note from FIG. 3 that η≅5.13 at j₂, and that η≅3.83 at j₁. The sin φ_(s)segments 1, 3 and 5 along the line 51 which intersects the η axis at j₂are shown in the inset curve 50. Similarly, the cos φ_(s) segments 2 and4 along the line 53 which intersects the η axis at j₁ are shown in theinset curve 52.

The preferred embodiment of the invention will be explained by nowreferring to the improved fiber optic gyroscope shown in FIG. 4 and tothe TABLE of FIG. 5.

The fiber optic gyroscope (FOG) of FIG. 4 includes directional couplers13A and 23A, a fiber optic coil 33A, an optical source 39A, aphotodetector 41A, an amplifier 43A and a piezoelectric transducer (PZT)45A, which are respectively similar to the elements 13, 23, 33, 39, 41,43 and 45 of FIG. 1 in both structure and operation. Hence, the elements13A, 23A, 33A, 39A, 41A, 43A and 45A of FIG. 4 require no furtherexplanation.

An oscillator or PZT driver 51 drives the PZT 45A sinusoidally at afrequency ω and PZT drive level or amplitude so as to add anon-reciprocal phase shift to the Sagnac phase shift φ, as discussed inrelation to FIG. 1. A portion of the output of the oscillator 51 is alsoapplied as a reference signal to a demodulator 53.

The amplified signal V at the output of the amplifier 43A is sent to thedemodulator 53 and to a peak-to-peak detector (PK-PK DET) 55. It will berecalled that this output signal V is given by Equation (4). Demodulator53, which can be comprised of mixing and filtering circuits (not shown),detects and extracts from the signal V the essentially DC values V(ω)and V(2ω), which respectively correspond to the fundamental and secondharmonic amplitudes contained in Equation (4). As shown in Equation (4).

    V(ω)=α.sub.1 I.sub.0 J.sub.1 (η) sin φ.sub.s (5a)

and

    V(2ω)=α.sub.2 I.sub.0 J.sub.2 (η) cos φ.sub.s (5b)

These two levels, V(ω) and V(2ω), are used to put the gyroscope intoeither STATE 1 or STATE 2 as a function of the Sagnac phase shift φ_(s).In each of STATES 1 and 2, one of the levels V(ω) and V(2ω) is used toprovide a feedback level which forces the PZT 45A to be driven at anappropriate associated drive level or amplitude (to be explained), whilethe other one of the two levels is used to sense the rotation rate byproviding an equivalent measure of the Sagnac phase shift φ_(s) between-π/4 and +π/4. This measure of the Sagnac phase shift φ_(s), inconjunction with the output of the peak-to-peak detector 55, is used toconstruct the least significant bits (LSB) of the rotation rate Ω. Themost significant bits (MSB) of the rotation rate are provided by a logiccircuit 57, which is also used to drive the gyroscope into one of thetwo STATES 1 and 2. This logic circuit 57 can be a computer or amicroprocessor.

The operation of the FOG of FIG. 4 in each of the STATE 1 and STATE 2modes of operation can best be understood by tracking the operation ofthe FOG as the input rotation rate is increased from zero.

-45°<φ_(s) <+45°

At low rotation rates of the platform, where the Sagnac phase shiftφ_(s) of the FOG is between -45° and +45°, the FOG is in STATE 1. Assumethat φ_(s) is increasing from 0°.

When the FOG is in STATE 1, an electronic switching network 59 passesthe V(ω) level (which corresponds to sin φ_(s)) to a rate sense arm 61and the V(2ω) level (which corresponds to cos φ_(s)) to an η feedbackarm 63. As Equation (5a) shows, the V(ω) level on arm 61 is low becauseφ_(s) is small at low rotation rates. As long as V(ω) remains low, alevel sense circuit 65 will output a signal to instruct the logiccircuit 57 to maintain the FOG in STATE 1.

The V(2ω) level on the η feedback arm 63 is passed through an electronicswitch 67 to an automatic gain control (AGC) circuit 69 which has itsoutput coupled to the oscillator or PZT driver 51. The AGC circuit 69 iscontrolled by signals from the logic circuit 57 to force the oscillator51 to develop its output signal amplitude to cause a preselected drivelevel η during STATE 1.

In contrast to the conventional FOG of FIG. 1, during STATE 1 the drivelevel η is chosen to be approximately 5.13 (rather than 1.84). This 5.13value of η is commonly referred to as j₂ or the zero of J₂ : J₂(5.13)=0. Driving the PZT 45A at η=j₂ is disadvantageous in that itreduces the overall system gain (compared to the FOG of FIG. 1), sinceJ₁ (1.84)=0.58 while J₁ (5.13)=0.34. However, there is an advantage instability that is offered by driving the gyroscope at this particularvalue of η. Any small drift in η away from j₂ will be amplified by theelectronics in a feedback network 84 which then provides a signed errorsignal to the AGC circuit 69 to return the output amplitude of theoscillator 51 back to a level to cause η to be 5.13. More particularly,if the amplitude of η is such that the V(2ω) level from the demodulator53 is not zero, the high open loop gain in the feedback network 84 willalter the amplitude of the PZT driver 51 until a zero output of V(2ω) isobtained. The feedback network 84 is comprised of the oscillator 51,transducer 45A, coil 33A, couplers 23A and 13A, photodetector 41A,amplifier 43A, demodulator 53, switching network 59, inverter 83,electronic switch 67 and AGC circuit 69.

As shown in Equation (5b), the zero output of V(2ω) can be due only tothe level η and not to φ_(s), since cos φ_(s) is almost equal to 1 forsmall rotation rates. As a consequence, the amplitude η is held fixed bythe above-described feedback network which holds V(2ω)=0 until φ_(s)changes to ≈±45°.

The V(ω) level on the rate sense arm 61 and the peak-to-peak voltage(2I_(o) of Equation (4) of the signal V from the peak-to-peak detector55 are both applied to a LSB (least significant bits) table or generator71. The table 71 can be a table lookup or arcsine calculator. Table 71normalizes the V(ω) level by dividing it by the peak-to-peak voltage.Then the resultant normalized V(ω) level, which is an arcsine function,is decoded by the table 71 to form or construct the least significantbits (LSB) in the digital rotation rate word representing Ω. Note thatthe arcsine function is most accurate in this range between -45° and+45°, and that the sensitvity of the gyroscope is only degraded to 0.707of its most sensitive value at the extreme limits of ±45°.

At this time the most significant bits (MSB) of the rotation rate wordrepresenting Ω are zero. The MSB of the rotation rate word are stored ina MSB (most significant bits) counter or register 73, which outputs azero at this time. The digital LSB output (φ_(LSB)) of the LSB table 71is passed through an electronic multiplexer unit (MUX) 75 to an adder77, which arithmetically sums that digital LSB output φ_(LSB) with thedigital MSB output φ_(MSB) of the MSB counter 73 to produce the rotationrate word Ω. The MUX 75 has the logical capability of inverting the signof the digital output φ_(LSB).

+45°<φ_(s) <135+°

Now assume that the rotation rate has increased such that φ_(s) >45°. Asφ_(s) increases, so does V(ω) which corresponds to sin φ_(s). When φ_(s)=45°, the amplitude of V(ω) is sufficiently high to cross a preselectedthreshold set by the level sense circuit 65. In response to this sensedamplitude of V(ω), the level sense circuit 65 outputs a signal toinstruct the logic circuit 57 to change the gyroscope of FIG. 4 fromSTATE 1 to STATE 2. Upon receiving this signal from the level sensecircuit 65, the logic circuit 57 sends control signals over a bus 79 tocause the switching network 59 to switch the roles of V(ω) and V(2ω). Asa result, V(2ω) (which corresponds to cos φ_(s)) is now applied to therate sense arm 61, while V(ω) (which corresponds to φ_(s)) is nowapplied to the η feedback arm 63. The logic circuit 57 also sendscontrol signals over a bus 81 to cause the AGC circuit 69 to approximatea new correct-driving level which forces the modulator 51 to develop itsoutput amplitude at or near a second preselected drive level such as toproduce η=j₁ for STATE 2. In addition, the logic circuit 57 incrementsthe count of the MSB counter 73 by a digital word corresponding to 90°,since the STATE of the gyroscope has been changed. If necessary, thelogic circuit 57 also sends a signal to the switching circuit 67 tocause the logic sign or polarity of the voltage level on the η feedbackarm 63 to be inverted by inverter 83 in order to apply negative feedbackto the AGC circuit 69. Finally, the logic circuit 57 sends a signal tothe MUX 75 to enable the φ_(LSB) from table 71 to be logically invertedand passed to the adder 77.

Such a need for changing the sign of φ_(LSB), when φ_(s) is between +45°and +135°, can be realized by referring to FIG. 5. When φ_(s), isbetween -45° and +45°, φ_(LSB) is between -45° and +45°, φ_(MSB) =0, andthe rate word Ω is simply equal to φ_(LSB). However, when φ_(s) isbetween +45° and +135°, φ_(LSB) is between +45° and -45°, respectively(as implicitly shown in FIG. 2) and φ_(MSB) =90°. At this time assumeφ_(s) =53°. Since V(2ω) (which is now on the rate sense arm 61)corresponds to cos φ_(s) (or cos 53°) and table 71 is an arcsinecalculator, the arcsine of cos 53° will produce a φ_(LSB) angle=37°.This 37° degree φ_(LSB) angle must be subtracted from the φ_(MSB) angleof 90° to produce a rotation rateφ corresponding to 53°. Thus, the logicsign of φ_(LSB) must be changed when φ_(s) is between +45° and +135°.

During STATE 2, the drive level η of the ω output from the oscillator 51is chosen to be ≈3.83. This 3.83 value of η is commonly called j₁ and isthe zero of J₁, such that J₁ (j₁)=0. In other words, as shown in FIG. 3,j₁ is the value of η such that J₁ (η)=0 when η=j₁. The value of η ischanged to j₁ in STATE 2 in order to obtain an appropriate error signalin the feedback network 84. If η deviates from j₁, the non-zero value ofV(ω) on the η feedback arm 63 will cause the elements in the feedbacknetwork 84 to force a change in the amplitude of the oscillator or PZTdriver 51 until the null at V(ω) is found.

φ_(s) >+135°

When φ_(s) >135°, the FOG of FIG. 4 reverts to STATE 1, the MSB counter73 is again incremented by 90° to hold a digital MSB number(representing φ_(MSB)) corresponding to an angle of 180°, V(ω) and V(2ω)again switch roles with V(ω) being passed to the rate sense arm 61 andV(2ω) being passed to the η feedback arm 63, the AGC circuit 69 iscontrolled by the logic circuit 57 to drive the PZT driver 51 to developa drive level η=j₂, the multiplexer 75 is controlled by logic circuit 57to pass the logical inversion of φ_(LSB) to adder 77 (as indicated inthe TABLE of FIG. 5). In addition, because of the sign change in cosφ_(s), the feedback signal of V(2ω) on the feedback arm 63 must alsochange sign. As mentioned before, the logic circuit 57 controls thissign change of the feedback signal on the arm 63 by enabling theelectronic switch 67 to pass the inversion of the feedback from theoutput of inverter 83 to the AGC circuit 69.

As the rotation rate continues to increase and cause φ_(s) to increase,the gyroscope switches from STATE 1 to STATE 2 and back again, asdiscussed before for other rotation rates. If the rotation rate weredecreased, the gyroscope would similarly switch between the STATES 1 and2, but would decrement the MSB counter 73 each time the STATE changed asthe rotation rate decreased.

The TABLE of FIG. 5 shows other ranges of φ_(s), with the associatedranges and the logic signs of φ_(LSB) (applied to MUX 75) during STATES1 and 2, the feedback logic signs applied to the electronic switch 67,and the values of φ_(MSB) and values of the rate word (Ω). The operationof the gyroscope of FIG. 4 over other ranges of φ_(s) can be readily bedetermined with the use of the TABLE of FIG. 5, the response curves ofFIG. 3 and the operation of the gyroscope over the ranges previouslyexplained.

Therefore, what has been described is an improved fiber optic gyroscopewith a substantial improvement in dynamic range and scale factorstability over a basic fiber optic gyroscope. The improved fiber opticgyroscope automatically switches the state of the gyroscope between twostates as a function of the rotation rate in order to keep thesensitivity and accuracy high for rotation rate sensing whilestabilizing the drive level η at a level above pi during each state.

It should therefore readily be understood that many modifications andvariations of the present invention are possible within the purview ofthe claimed invention. It is therefore to be understood that, within thescope of the appended claims, the invention may be practiced otherwisethan as specifically described.

What is claimed:
 1. A fiber optic gyroscope system comprising:a fiberoptic coil for counterpropagating substantially equal intensity firstand second beams therethrough to develop first and second output beamswith a relative Sagnac phase shift therebetween; means coupled to saidfiber optic coil being responsive to a drive signal for providing amodulation to phase shift the Sagnac phase shift between said first andsecond output beams; an optical source for producing an input beam;directional coupler means responsive to the input beam for providingsaid substantially equal intensity first and second beams forcounterpropagation through said fiber optic coil, said directionalcoupler means also combining said first and second output beams to causea resultant interference pattern to be developed; photodetection meansresponsive to said resultant interference pattern for developing adetected signal indicative of the intensity of said resultantinterference pattern; means for demodulating said detected signal todevelop first and second voltage levels respectively proportional tosine and cosine components of said detected signal; feedback meansresponsive to said second signal during a first state for providing afirst stabilized drive signal to said providing means and being furtherresponsive to said first signal during a second state for providing asecond stabilized drive signal to said providing means; rate sensingmeans responsive to the most sensitive portion of said first signalduring said first state and to the most sensitive portion of said secondsignal during said second state to accurately sense the rotation rate;logic means coupled to said demodulator means and to said rate sensingmeans for selectively changing the state of the gyroscope between state1 and state 2 as a function of the rotation rate in order to provide themore sensitive and accurate one of said first and second voltage levelsto said rate sensing means for sensing the rotation rate and in order toprovide the higher amplitude of the second and first voltage levels tosaid feedback means to stabilize the drive signal being applied to saidproviding means.
 2. The system of claim 1 wherein said logic meanscomprises:switching means responsive to a first control signal forrespectively passing the second and first voltage levels to saidfeedback means and said rate sensing means, and being further responsiveto a second control signal for respectively passing the first and secondvoltage levels to said feedback means and said rate sensing means; levelsensing means coupled to said rate sensing means for generating athreshold signal whenever the amplitude of that one of said first andsecond threshold levels being applied to said rate sensing means exceedsa predetermined threshold level; and a logic circuit coupled to saidlevel sensing means being responsive to each threshold signal forchanging the state of the gyroscope from one of the states 1 and 2 tothe other and for alternatively generating one of the first and secondcontrol signals each time that a threshold signal is generated.
 3. Thesystem of claim 2 wherein said feedback means comprises:an oscillatorfor providing said drive signal to said providing means; automatic gaincontrol means responsive to the second signal from said switching meansand to a first amplitude signal from said logic means for developing afirst stabilized drive signal during said first state and being furtherresponsive to the first signal from said switching means and to a secondamplitude signal from said logic means for developing a secondstabilized drive signal during said second state.
 4. The system of claim2 wherein said rate sensing means comprises:first means responsive tothat one of the first and second signals being applied thereto fordeveloping a least significant bit number between -45° and +45°; secondmeans for incrementing its count by 90° each time that said logic meanschanges states in order to develop a most significant bit number;andmeans responsive to a logic sign signal from said logic means and tosaid least and most significant bit numbers for developing a numberrepresentative of the rotation rate.
 5. The system of claim 4 whereinsaid feedback means comprises:an oscillator for providing said drivesignal to said providing means; automatic gain control means responsiveto the second signal from said switching means and to a first amplitudesignal from said logic means for developing a first stabilized drivesignal during said first state and being further responsive to the firstsignal from said switching means and to a second amplitude signal fromsaid logic means for developing a second stabilized drive signal duringsaid second state.
 6. The system of claim 1 wherein the amplitude ofsaid modulation to phase shift the Sagnac phase shift between said firstand second output beams is greater than π.
 7. The system of claim 4wherein the amplitude of said modulation to phase shift the Sagnac phaseshift between said first and second output beams is greater than π. 8.The system of claim 7 further including:means responsive to saiddetected signal from said photodetection means for developing apeak-to-peak signal therefrom; said first means normalizing that one ofthe first and second signals being applied thereto by dividing thatsignal by said peak-to-peak voltage and then utilizing that normalizedone of the first and second signals being applied thereto to developsaid least significant bit number between -45° and +45°.